With evolving technology, enabling a single operator to control multiple robots or a robotic swarm is becoming technically feasible. However, policy and regulatory frameworks do not yet address these scenarios, and we do not yet have a strong understanding of the impact of controlling large numbers of robots on human situation awareness. This paper addresses this gap through an empirical study that systematically degrades interface information to assess its impact on operator situation awareness and task performance in a human-swarm interaction scenario. Using a tablet-based interface, participants guide a robotic swarm through a simulated spreading fire task, and situation awareness and task performance are monitored. Results reveal that degrading information leads operators to focus on a single robot, often neglecting the rest of the swarm, a narrowing of attention that raises concerns for single-operator-multi-robot operations. Additionally, only certain information types were found to significantly affect situation awareness and performance, suggesting that some elements are critical while others are not. These findings underscore the need for regulatory support mechanisms such as secondary operators and call for flexible, task-specific policies that prioritise the most critical information elements in multi-robot operations.